In vitro diagnostic tests to identify and treat diseases have become common tools in hospitals, homes and physician's offices. Biological fluids such as blood, urine or cerebrospinal fluids, which may at times contain blood, are the most frequently employed biological samples for such tests. Of these, blood is the most commonly used. However, for most testing applications, blood must be separated into plasma or serum prior to testing.
Blood separation technologies can generally be grouped within three categories—centrifugation, filtration, and solid-phase separation.
Centrifugation is generally used to separate cellular components from serum or plasma because (1) centrifugation can separate cellular components from serum or plasma at an efficiency of greater than 95%; (2) centrifuges do no require highly trained personnel to operate; and (3) centrifugation allows concurrent processing of multiple samples in a relatively short time. However, some difficulties exist with centrifugation. For example, centrifuges are expensive, require the need for multiple steps (e.g., aliquoting through the use of precision pipette which can vary depending upon the technique of the operator), and are often unavailable at points of care such as a home, school, or bedside.
Filtration techniques to separate components of blood can be performed in a variety of manners. U.S. Pat. No. 4,987,085 to Allen et al., for example, describes a filtering system with descending pore size using a combination of glass fiber membranes and cellulose membranes. U.S. Pat. No. 4,753,776 to Hillman et al. discloses a glass microfiber filter using capillary force to retard the flow of cells. U.S. Pat. No. 4,256,693 to Kondo et al. discloses a multilayered chemical analysis element with filter layers made from at least one component selected from paper, nonwoven fabric, sheet-like filter material composed of powders or fibers such as man-made fibers or glass fibers. U.S. Pat. Nos. 3,663,374 and 4,246,693 disclose membrane filters for separating plasma from whole blood and U.S. Pat. Nos. 3,092,465, 3,630,957, 3,663,374, 4,256,693, 4,246,107, 4,330,410 disclose further filtration systems, some of which make use of small-pore membranes.
In general, filtration can be favorable because filtration reduces the volume of blood required to only a few drops. However, significant amounts of plasma may be retained and lost in the filters of known devices, and low concentrations of analytes derived from small volumes of blood are difficult to detect.
Solid-phase separation involves a surface having the ability to bind to a target. In other words, the surface effectively acts to immobilize and remove the target from the sample.
One type of solid-phase separation is magnetic separation, in which a target is captured by magnetically attractable beads. U.S. Pat. No. 7,214,544 to Poirier et al. describes an apparatus and method of blood separation using magnetic beads that are coupled to an affinity marker wherein the target is separated from the rest of the fluid using magnetic force and an automatic mechanical force. Furthermore, U.S. Pat. No. 6,291,249 to Mahant et al. describes the use of antibodies that are coupled to the surfaces of paramagnetic beads. One advantage of using magnetic separation is the absence of physical barriers which tends to make the separation relatively gentle. However, a major limitation with applying known magnetic separation is that multiple anti-ligands are required to remove all of the various types of cells and sub-cellular particles. Moreover, lack or absence of ligands on the cells due to pathological conditions, genetic diseases or genetic variations or life cycles of cells generally reduce the efficiency with which the anti-ligands bind with the target cells.
Thus, there is still a need to provide improved methods and apparatus for separating blood into its constituent parts, and especially for separating plasma or serum from whole blood.